Methods for controlling motors have been under development for some time. In particular, there have been advances in the design and development of a regeneration circuit used to control the charging of a battery of an electrically powered vehicle by converting vehicle mechanical energy into electrical energy. However, prior art controls in this area have proven unsatisfactory because of their low utilization of the vehicle mechanical energy.
Now referring to FIG. 1, in the prior art it is common to use a pulse width modulation technique (hereinafter PWM) for controlling the electric motors of electric vehicles. In such an arrangement, motor 10 is powered by battery 12 to delivery mechanical power 14 to the drive shaft of the vehicle. Motor 10 is pulsed on and off by the presence of signal 16, generated by drive control, or microprocessor 18, to semiconductor switch 20, which is typically a power MOSFET. The arrangement shown in FIG. 1 is known as a class B chopper circuit. When switch 20 is not switched on (motor coasting), steering diode 22 and resistor 24 dissipate any energy generated by the motor 10. As shown in FIG. 1B, switch 20 is closed (motor on or driving) when signal 16 is high, shown by motor "on" interval 26; similarly, switch 20 is open (motor coasting) when signal 16 is low, shown by motor "off" interval 28. Only during vehicle braking is energy (which is generated by motor 10) returned to the battery 12 (circuit for this is not shown). This brake-only regeneration is used due to the problem of being able to guarantee that, if a semiconductor switch was used in place of steering diode 22, the upper and lower semiconductors switches would never be on at the same time. Having the upper and lower semiconductor switches on simultaneously can cause catastrophic failure of the switches (i.e., the battery will be effectively short-circuited). Some progress has been made toward alleviating this problem, in general, by the development of the concept of "deadtime." Deadtime is a time interval injected prior to and after the drive signal for either the upper or lower semiconductor switches so that both of the switches won't be "on" at the same time.
A further problem encountered by the prior art involves the dependency of power regeneration on the duty cycle of the PWM signal 16. That is, for a given period of signal 16 (the sum of "on" interval 26 and "off" interval 28), the regeneration time (interval 28) is inversely proportional to the motor drive or "on" interval 26. Since a torque braking effect is imparted by motor 10 when operating in a power regeneration mode, this lack of flexibility in the control of the power regeneration time results in relatively undesirable drivability characteristics in electric vehicles, when compared to vehicles equipped with internal combustion engines.
Thus there is a need to provide an improved apparatus and method for power regeneration in an electric vehicle motor drive that is directed to overcoming one or more of the problems as set forth above.